1. Field of Invention
The invention is related to an actuator comprising a housing, a motor, an actuating member and a screw mechanism providing a linear movement of the actuating member with respect to the housing in response to a rotational movement of the motor, said screw mechanism comprising a screw and a nut one of which is rotatably supported with respect to the housing.
2. Description of Related Art
Such actuator is known from WO-A-9603301. The actuator comprises a screw mechanism, consisting of a screw and a nut which engage each other by means of rollers having circumferential grooves. Such actuator provides a well defined axial displacement, and also a certain degree of reduction. Thus, a fairly, small axial displacement is obtained in response to a considerable rotation of the screw. Consequently, the screw may exert a considerable axial force on the actuating member.
The proper operation of the screw mechanism can be assured as long as the screw, nut and rollers are subject to a well-defined, axial load. In such case, all components are loaded in accordance with their design requirements; moreover, the load may then reach a considerable level without causing damage to the screw mechanism.
A very unfavorable case however occurs when the screw mechanism is subjected to loadings having a transverse or radial component. The screw, nut and rollers are not designed to accommodate the transverse load components, and will be damaged.
The same occurs in case the actuator is loaded by a bending couple, which is always associated with such transverse forces.
The object of the invention is to provide an actuator as described before, in which the problems related to transverse or radial loadings are circumvented or at least alleviated. This object is achieved in that the screw mechanism and the actuating member engage each other through a resilient intermediate pressure means. The resilient intermediate pressure means is able to transfer the required actuating force from the screw mechanism onto the actuating member. Thus, its stiffness in axial direction should be rather high. In particular, the stiffness should be maintained at a level where the required force/displacement relationship still provides the possibility to obtain the desired actuating force.
On the other hand, said resilient intermediate pressure means is not as stiff as a direct connection between the screw mechanism and the actuating member. This adapted stiffness has the advantage that extreme loadings, which have a certain transverse component or bending moment, are not directly and fully transmitted towards the screw mechanism. The resilient aspect of the force transmission between the screw mechanism and actuating member makes these transverse or bending loadings less severe or even absent.
According to a first possible embodiment of the invention, the screw mechanism and the actuating member engage each other through spring elements. The spring elements may provide an asymmetric stiffness distribution with respect to the axis of the screw mechanism. The asymmetric character of the spring elements may be obtained in several ways, e.g. by more or stiffer springs at one side of the axis of the actuating member and the screw mechanism than at the opposite side.
The advantage of an asymmetric layout of the spring elements is that it may anticipate an asymmetric loading pattern under full loading. An example of a non-aligned loading, which increases with the load level, is to be attributed to flexing of the claw piece in the application of an actuator in a disc brake.
According to a second possibility, the screw mechanism and the actuating member engage each other by means of a resilient pressure pad.
Such pressure pad may have a relatively small thickness compared to its lateral dimensions. It can therefore easily be accommodated between the actuating member and the screw mechanism, within narrow space constraints.
The pressure pad preferably comprises two generally parallel wall members, said wall members being mutually connected along their circumference and enclosing a closed internal space.
The actuating member may be carried out as a piston, said piston being accommodated in a cylinder which forms a bore in the housing of the screw mechanism. Thus, the intermediate resilient pressure member is supported against the inside of the piston head.
According to a further development, the resilient intermediate pressure means engages a load measuring device. In particular, the load measuring device senses the pressure of the fluid.
The pressure developed in the fluid provides a reliable measure of the force exerted by the actuator. Possible a non-axial or excentric loadings will not hamper the measurement of the overall axial loads to which the actuator is exposed. Thus, a reliable measurement is obtained.
If convenient, the internal space of the pressure pad is connected to a measuring channel, the free end of said channel being provided with the load measuring device. Thereby, the actual measurement, e.g., by a piezoelectric sensor, may be carried out a particular location which is for instance shielded from the area where the actuating member is located (heat, moisture).
The invention is also related to a brake caliper comprising a housing, a motor, an actuating member and a screw mechanism providing a linear movement of the actuating member with respect to the housing in response to a rotational movement of the motor, said screw mechanism comprising a screw and a nut, one of which is rotatably supported with respect to the housing. The screw mechanism and the actuating member engage each other through a resilient intermediate pressure means for pressing the brake pads onto the brake disc.
The resilient intermediate pressure means of the brake caliper may engage a load measuring device as well. The internal space of the pressure pad is connected to a measuring channel, the free end of said channel being provided with the load measuring device.
The actual load measuring device (e.g. a piezoelectric sensor) may now be situated at a location where the influence of the heat developed by the friction between the brake pads and the brake disc, is reduced.
The measuring channel may extend through the screw of the screw mechanism, the load measuring device being situated at the end of the screw which is opposite the pressure means. Furthermore, the load measuring device comprises a load cell or load sensor, the electric signal line thereof extending through thee internal space of the motor towards a connector or the like on the housing.
Preferably the fluid is a temperature resistant thermal oil.